After MDEV-12353, the consistency check that I originally added for
commit 1b9fe0bb
(InnoDB Plugin for MySQL 5.1) started randomly failing.

It turns out that the IMPORT TABLESPACE code was always incorrect:
it did not update the (redundantly stored) tablespace ID
in index tree root pages. It only does that for page headers
and BLOB pointers.

PageConverter::update_index_page(): Update the tablespace ID
in the BTR_SEG_TOP and BTR_SEG_LEAF of index root pages.